This invention relates to ion etching, more specifically to an ion source and an ion etching process effectively used for the etching of semiconductor devices.
Recently, the method for etching patterns of semiconductor integrated circuits has been being switching from the wet process utilizing chemical solutions to the dry process such as plasma etching. This is attributable to the fact that the dry process surpasses the wet process in some points; improved accuracy, simplicity of processing stages achieved by automation, pollution-free processing without producing waste fluid, etc.
For the dry etching process, there are known sputtering etching and ion etching, besides the plasma etching. The plasma etching among these processes, which is performed in plasma obtained by impressing high frequency to a gas such as fluorocarbon, is not directional but isotropic, so that an undercutting will be formed under a mask in the same manner as the case of the wet etching process (FIG. 1 shows an undercutting). Owing to such undercutting, the plasma etching can provide only a pattern with a thickness of about 4 .mu.m, so that it can hardly be applied to the manufacture of ultra LSI's which requires superfine processing to provide a pattern width of 1 to 2 .mu.m.
In the sputtering etching, on the other hand, gas plasma is produced by means of high frequency, and a workpiece to be etched, which has previously been cooled, is disposed on the cathode side, and etched by sputtering ions that are accelerated by cathode drop voltage. According to the sputtering etching, therefore, reactive ions run against the workpiece with directivity, thereby improving the undercutting of the mask. In this case, however, the etching mask touches an atmosphere of gas plasma, so that it will be retreated during the etching process, and hence the etching pattern will be forced to extend.
The ion etching may be regarded as a dry etching process that obviates the above-mentioned defects of the plasma etching and sputtering etching. According to the ion etching process, a thin gas at a pressure of e.g. 10.sup.-4 torr is made into plasma, and etching is performed by means of reactive ions that alone are taken out of the plasma. The mean free path of the reactive ions is wide enough to prevent the undercutting of the mask, and the mask will not be retreated because the to-be-etched workpiece is kept from the gas plasma. Thus, the ion etching may be regarded as highly effective for the superfine processing of semiconductor devices.
FIG. 3 shows a prior art electron impulse type (Koufman type) ion source incorporated in an apparatus for such ion etching. This ion source consists of a vacuum container 31, a filament 33 and a cylindrical anode 34 of e.g. aluminum disposed in the vacuum container 31, a grid 35 of e.g. molybdenum disposed opposite to the filament 33, a mesh-like cathode 38 facing the grid 35 with an insulating member 37 between, and a coaxial coil 36 surrounding the vacuum container 31. The operating principle of the ion source is as follows. Heat electrons emitted from the filament 33 move toward the grid 35 in accordance with a voltage of e.g. hundreds of volts which is applied between the cylindrical anode 34 and the grid 35. At the same time, the heat electrons are subjected to the action of a magnetic field created by the coaxial coil 36, after all moving in a cycloid. Then, the heat electrons run against gas molecules of a gas to be made into plasma at a pressure of approximately 10.sup.-4 torr that is introduced through a gas inlet 32 in the vacuum container 31, thereby forming gas plasma. Positive ions out of the gas plasma are extracted and accelerated by the mesh-like cathode 38 to form an ion beam 39. With such ion source, there may be provided a large beam diameter, as well as a large current flow. Presently, Koufman type ion souces with a diameter of 10 inches are being developed.
However, the tungsten filament or heat cathode, which is used with the above-mentioned ion source, is etched to be cut or broken by introduced reactive gases, such as CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8, CHF.sub.3, SiF.sub.4, etc., in a few minutes to tens of minutes, thereby interrupting electric discharge. Thus, the ion source of this type has only too short a life to be industrially applied to the etching process for semiconductor devices.
As an improved version of such heat cathode type ion source, there has recently been developed a magnetron type ion source having no electron emitting means and utilizing space charges. FIGS. 4A and 4B show an outline of such ion source. In this ion source, a flat anode 41 of e.g. stainless steel and a box-type cathode 42 of e.g. mild steel are disposed opposite to each other. When a voltage is applied to both these electrodes by an external direct current power source, an electric field is formed between them. A circular slit 44 is cut in a side of the cathode 42 that faces the anode 41. The direction of the electric field in the vicinity of the slit 44 is indicated by arrow A. Inside the box-type cathode 42, on the other hand, there is disposed a coaxial coil 45. The direction of a magnetic field created by the coil 45, in the vicinity of the slit 44, is indicated by arrow B. Namely, an orthogonal electromagnetic field is formed in a closed loop near the circular slit 44, and the electrons, which exist as the spaced charges from the beginning, circulate along the closed loop. Hereupon, when a gas to be made into plasma is introduced through a gas inlet 47 of a vacuum container 46, the gas molecules run against the circulating electrons to form gas plasma. An opening 48 for ion exhaust is bored through the bottom of the box-type cathode 42, and positive ions in the gas plasma are accelerated and drawn out to the exterior through the opening 48 by the electric field.
Utilizing no filament, the ion source of this type involves no problem as regards the life span, despite the use of the reactive gases, though it still is subject to the following defects:
(1) The use of an electric magnet requires a large housing space, thereby making the apparatus unreasonably large-sized.
(2) The electric magnet requires a power source for its exclusive use.
(3) Since the coaxial coil cannot help being disposed around the looped slit, most of the magnetic energy is consumed outside the coil, making it difficult to obtain an intensive magnetic field in the vicinity of the looped slit.
(4) It is hard to discharge water and gas sticking to the coil surface, and it takes a long time to improve the degree of vacuum.